A multiple-input-multiple-output (MIMO) communications network comprises at least one base transceiver station (BTS) equipped with multiple antennas and a plurality of mobile stations (MSs), at least one of which has multiple antennas. Beamforming is a technique that utilizes multiple antennas to create beamformed channels between a BTS and an MS. Compared with a typical MIMO channel, a beamformed channel provides higher throughput and better diversity, which effectively improve the performance of a communications network.
The BTS calculates beamforming weighting vectors using channel information so as to create beamformed channels. More specifically, the BTS computes downlink beamforming weighting vectors using uplink sounding signals received from the MS. The transmission of sounding signals uses system resources. Therefore, the amount of system resources reserved for the transmission of sounding signals determines the number of MSs that can be supported in a wireless communications network.
One way to reduce the overhead incurred due to the transmission of sounding signals is that a BTS can utilize other sources, in addition to sounding signals, to obtain channel information for the computation of beamforming weighting vectors. One of these sources is uplink traffic from an MS. However, uplink traffic is of no use if certain situations occur. For example, if downlink and uplink traffic is not balanced or if there is a mismatch between the frequencies of downlink and uplink channels, channel information cannot be extracted from uplink traffic. Therefore, in order to fully utilize the multiple antennas on a BTS, the wireless communications network needs to incorporate other MIMO techniques which do not require channel information.
Space-time coding (STC) and cyclic-delay diversity (CDD) are two of the MIMO techniques that do not need channel information to construct signal streams for a MIMO channel. The STC scheme improves the performance of a wireless communications network by transmitting multiple redundant copies of a data stream. By contrast, the CDD scheme improves the network performance by transforming spatial diversity into frequency diversity by using different delays among transmit antennas. More specifically, multiple antennas transmit the same data stream with different cyclic delays. Generally speaking, the performance of a wireless communications network employing the STC scheme is inferior than that of a network employing a beamforming technique because the symbol rate is reduced in the orthogonal STC scheme when it is applied to an MS with more than two antennas in an antenna array.
None of the techniques mentioned above for utilizing MIMO channels can operate alone without leaving some issues unsolved. Therefore, a desirable way would be for a wireless communications system to adaptively switch between different transmission schemes depending on the availability of channel state information (CSI).